Pomeranchuk effect

The Pomerantschuk effect (by the Russian physicist Isaac Pomerantschuk ) is a compression refrigeration along the ³ He - melting curve as a result of density anomaly .

Liquid ³ He can be below about T  = 0.3  K by (= -272.85 ° C) heat supply to solidify (positive heat of solidification , usually negative). The heat required for this is withdrawn from the surrounding system, e.g. B. by adiabatic compression , it is cooled, hence the name compression cooling. (In the case of conventional liquids without anomalies or also with ³ He above 0.3 K, on ​​the other hand, compression leads to heating.)

pT diagram of ³ He
( logarithmic temperature scale );
the melting curve is the boundary between a solid ( solid ), and normal fluid ( normally liquid )

The Pomerantschuk effect is based on the fact that the slope of the melting curve of  ³ He in the pT diagram ( pressure , temperature ) is negative at T <0.3 K:

${\displaystyle {\frac {\mathrm {d} p}{\mathrm {d} T}}<0}$

From the Clapeyron equation

${\displaystyle {\frac {\mathrm {d} p}{\mathrm {d} T}}={\frac {\Delta s}{\Delta v}}}$

With

${\displaystyle \Delta s}$- Change in entropy during phase transition

${\displaystyle \Delta v}$- Change in volume

follows

${\displaystyle \Rightarrow {\frac {s_{\text{fl}}-s_{\text{fest}}}{v_{\text{fl}}-v_{\text{fest}}}}<0}$

and with

${\displaystyle v_{\mathrm {fl} }>v_{\text{fest}}}$

follows

${\displaystyle \Rightarrow s_{\text{fl}}<s_{\text{fest}}}$

d. H. the solid phase of ³ He has a higher entropy than liquid ³ He at T <0.3 K (usually the other way round).

Individual evidence

1. ↑ Isaak Pomerantschuk: On the theory of liquid 3 He . In: Zh. Eksp. Teor. Fiz. tape 20 , no. 919 , 1950.